The invention relates to a method for activating an asynchronous motor with electronic switches for rotational speed change.
Typically, asynchronous motors are used for applications of a small power, for example up to two hundred Watts, as are applied for the drive of centrifugal pumps, for example heating circulation pumps and likewise. Thereby, it is mostly the case that monophase and biphase asynchronous motors are fed by the mains in a monophase manner. The rotational speed of these motors thereby is dependent on the mains frequency.
For the control of the rotational speed, it is known to connect parts of the auxiliary winding in series with the main winding. It is counted as belonging to the state of the art to connect frequency converters in series for a quasi stepless change in the rotational speed, but these are complicated and expensive. A change of the rotational speed may also be created by way of a suitable an activation of electronic switches, typically triacs, with which a phase-angle control is effected. The problem of such phase-angle controls lies in the fact that the lag angle, thus the angle after which the switch is switched on again after running through the zero point of the supply voltage, in order to supply the motor with voltage, may only be varied within tight limits. Small lag angles have little influence on the effective value (RMS-value) of the voltage, whereas large lag angles influence the power factor of the motor in an unallowable manner. This problem is compounded with an increasing power of the motor. Moreover, the power which is brought into the motor is also reduced with an increasing lag angle.
Alternatively to this, it is counted as belonging to the state of the art to carry out the rotational speed control with the help of a pulse-cascade control. Here, half-waves of the supply voltage of the main winding are cut away by way of the triacs, by way of a switch lying in series with the main winding being suitably activated in an opening manner. As can be understood, one may not filter out an infinite amount of part-waves of the supply voltage, since then the motor would not be adequately supplied, and would no longer run at a constant speed. This, in particular with wet running centrifugal pump assemblies, may lead to the undesirable formation of noise, which is not desirable on account of the fluid coupling into the conduit network, thus for example into the heating installation. This effect too increases with an increasing motor power.
A toggling method, with which the main winding is connected in series to a part of the main winding, depending on the rotational speed to be activated, is much more favourable as far as this is concerned. An almost infinite control of the rotational speed may be effected by way of this, depending on the number of winding parts and of the switches which are assigned to these. This, depending on the rotational speed to be set, however requires a constant switching between the winding parts which are connected in series to the main winding. This method, particularly with regard to the formation of noise and the influence on the power factor, is much more favourable than the previously described phase-angle control or pulse cascade control which were described previously, but comes up against its limits in particular with motors of a greater power, since current peaks arise in the winding with each switching, which burden the mains and the motor.
Against this background, it is the object of the invention to provide a method for activating an asynchronous motor, which largely avoids the previously mentioned disadvantages, and may be applied in particular also with motors of a greater power.